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Wood you believe it?

18 Oct 16 Scientists have developed a method of making wood transparent. David Taylor reports

It sounds like an April Fools’ joke – in fact, the BBC News website recently declared that it should be one – but transparent wood is not a joke, it’s a real thing.

Research scientists at the KTH Royal Institute of Technology in Stockholm, Sweden, recently announced that they had succeeded in creating “optically transparent wood” through a process in which the light-absorbing lignin component is chemically removed and the resulting “delignified nanoporous wood template” impregnated with pre-polymerised methyl methacrylate resin. By matching the refractive index of the MMA resin to that of the delignified cellulose fibres, the scientists produced optically transparent wood in thicknesses up to 8mm.

Lars Bergland, who led the research, explains that “when the lignin is removed, the wood becomes beautifully white. But because wood isn’t naturally transparent, we achieve that effect with some nanoscale tailoring”.

A paper (“Optically Transparent Wood from a Nanoporous Cellulosic Template: Combining Functional and Structural Performance”) published by the Stockholm team in the American Chemical Society journal Biomacromolecules, notes that cellulose fibres have already been used to create transparent paper through cellulose dissolution and regeneration, polymer impregnation or decreasing the cellulose fibrous diameter to the nanoscale.

Japanese scientists have even done something similar with crab-shell, removing the inorganic calcium carbonate and the protein ‘glue’ and re-impregnating the remaining nanoporous chitin structure with a transparent resin.

The Swedish team took samples of balsa wood and extracted the lignin with a solution of sodium chlorite with an acetate ‘buffer’ at 80oC. After the lignin had been dissolved, the samples were washed in de-ionised water and then dehydrated using ethanol and acetone.

The remaining nanoporous wood template was impregnated with the prepolymerised MMA solution under vacuum for 30 minutes before being baked in an oven at 70oC for four hours.

The refractive index of the prepolymerised MMA was carefully matched to that of the delignified cell walls to maximise transparency. Delignified wood, though white, is not transparent because light is scattered by the difference in refractive index at the interface between the cell walls and the air.

Replacing the air with a resin that has the same refractive index as the cell walls renders the material transparent.

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This is all very clever, but why bother? Why go to all the trouble of making wood transparent when we already have glass and various transparent plastics at our disposal?

The answer, it seems, lies in energy efficiency and cost. The built environment accounts for 30 – 40% of total energy consumption, says the research team. Solar energy is free, inexhaustible and clean and hence “light-transmitting buildings can also contribute to reduced demand since artificial light can be partially replaced by natural light”.

Wood, the researchers point out, is the most widely used biological building material and has the advantage of having good strength, low density and low thermal conductivity. It is also relatively cheap. If you can enhance these qualities by adding something new – in this case, transparency – you broaden the range of applications for the material.

Transparent wood is more thermally-efficient than glass and its main component – cellulose – is renewable. After all, it’s made out of trees with only relatively little synthetic resin added.

Different types of wood will yield products of different transparency and ‘haze’ (the amount of light diffusion within the material). The Stockholm team has achieved transparency of up to 85% and haze of 71% in 1.2mm-thick wood samples.

“This high haze is attractive in solar cell applications,” explain the scientists. “Light will be trapped in the solar cell for a longer time due to light scattering caused by the wood tissue. Longer trapping time means better interaction between light and the active medium which can lead to better solar cell efficiency.”

The combination of the PMMA and delignified wood template produced a strong, tough material – twice as strong as neat PMMA, say the researchers. “Therefore, this nanostructured biocomposite is both a structural (mechanical properties) and a functional (optically transparent with haze) material,” concludes the team’s report.

“Optically transparent wood is an excellent candidate for lightweight and low-cost structures in light-transmitting buildings and for transparent solar cell windows.”

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